The invention relates to a method and apparatus for restoring ink jet printhead performance following a period in which the printhead has been capped. More particularly, the invention is directed to a method and apparatus for printhead the printhead, while in a capped position, by applying at least a first full tone firing pattern to the printhead followed by a second, partial tone, firing pattern.
An ink jet printer of the so-called xe2x80x9cdrop-on-demandxe2x80x9d type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink may be contained in a plurality of channels where power pulses are used to cause the droplets of ink to be expelled, as required, from orifices or nozzles at the ends of the channels.
In a thermal ink jet printer, the power pulses that result in a rapidly expanding gas bubble to eject the ink from the nozzle are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by voltage pulses (firing) to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in that particular channel and ink bulges from the channel orifice. At that stage, the bubble begins to collapse. The ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel orifice and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The nozzles in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicularly to the line of carriage movement, by a distance equal to the width of the printed swath. The carriage is then moved in the reverse direction to print another swath of information.
It has been recognized that there is a need to maintain the ink ejecting nozzles of an ink jet printer, for example, by periodically cleaning the orifices when the printer is in use, and/or by capping the printhead when the printer is out of use or is idle for extended periods. The capping of the printhead is intended to prevent the ink in the printhead from drying out. There is also a need to prime a printhead before use, to ensure that the printhead channels are completely filled with ink and contain no contaminants or air bubbles and also periodically to maintain proper functioning of the nozzles. This priming procedure is referred to as restoring the printhead to optimum printing status. Maintenance and/or priming stations for the printheads of various types of ink jet printer are described in, for example, U.S. Pat. Nos. 4,855,764; 4,853,717 and 4,746,938 while the removal of gas from the ink reservoir of a printhead during printing is described in U.S. Pat. No. 4,679,059.
One priming method for removing dried contaminants is to utilize a full tone firing in which all of the resistor heaters of a printhead are sequentially addressed (fired) once, or a number of times. The first resistor in a row is pulsed with each adjacent resistor being pulsed until all the resistors in a row have been pulsed, the complete row pulsing is referred to as a stroke. The printhead may be addressed by a plurality of full tone firings or strokes. U.S. Pat. No. 4,970,527 discloses such a system in which full tone priming is accomplished after a predetermined amount of time has elapsed since the last printing command.
A problem with this type of prior art full tone priming method is that because of the high frequency firing, the temperature of the printhead substrate heats up very quickly creating two undesirable phenomena. One is an increase in the number of air bubbles formed in the printhead ink reservoir. These air bubbles, if not removed, can create undesirable defects in the output prints. A second is that the printhead, if rapidly returned to the print mode, begins print operation at an undesirably high temperature, resulting in stress to the printhead substrate.
It is, therefore, an object of the invention to restore an ink jet printhead to an optimum print state by an improved priming operation which results in elimination of contaminants at the printhead nozzles while suppressing the formation of air bubbles in the printhead reservoir.
It is a further object of the invention to restore the printhead to a print mode at a lower, less stressful, temperature level.
It is a still further object to perform the improved priming operation at an optimum time period following initiation of a capping routine.
These, and other objects of the invention, are accomplished by applying a full tone firing pattern to the resistors of a printhead following a predetermined event, in the preferred embodiment, a capping function. The full tone firing pattern is then followed by a fractional tone firing pattern. For a fractional, or partial tone firing, the first resistor in a row is pulsed, followed by pulsing of non-adjacent resistors in a specified sequence. The first stroke, thus, fires only some of the resistors in the row. The second and subsequent firings are directed to resistors previously not fired, again in a preset sequence. By using the combination of a full tone firing followed by a partial tone firing, it has been found that air bubble formation is reduced. As a further desirable result, the fractional tone firing pattern produces a xe2x80x9ccool downxe2x80x9d period for the printhead before it is returned to print operation, resulting in less stress on the printhead and increased printhead life.
More particularly, the present invention relates to a method of restoring a printer, the printer including a printhead with a plurality of transducers and associated channels and ink ejecting nozzles, comprising the steps of:
moving the printhead into a capping condition,
addressing the transducers with a full tone firing sequence to create a, full tone priming where ink is ejected from said nozzles in a high density pattern and
addressing the transducers with a partial tone firing sequence to create a partial tone priming where ink is ejected from said nozzles in a low density pattern and whereby the printhead nozzles are cleared of contaminants during application of the full tone firing, and air bubbles are removed from the printhead during application of the partial tone firing.